Image forming apparatus

Abstract

To provide an image forming apparatus that can accurately estimate the state of deterioration of two-component developer with a simple method.SOLUTION: An image forming apparatus comprises image carriers, electrifying devices, developing devices, a developing voltage power supply, an electrification voltage power supply, a current detection unit, and a control unit. The developing devices each have a developer carrier that carries two-component developer including carrier and toner. The developing voltage power supply applies, to the developer carriers, a developing voltage including a DC voltage and an AC voltage. The control unit executes developer deterioration estimation processing of causing the current detection unit to detect a developing current when the developing voltage power supply applies the developing voltage to the developer carriers while changing the DC voltage between two different levels, with the image carriers being electrified to have a predetermined surface potential, and estimating the degree of deterioration of the two-component developer on the basis of the amount of change of the developing current between two levels.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an electrophotographic image forming apparatus, and more particularly to a method for estimating deterioration of a developer and a surface potential of a photosensitive member. [Background technology]

[0002] In conventional electrophotographic image forming apparatuses, an image carrier, such as a photosensitive drum having a photosensitive layer formed on its surface, is charged to a predetermined surface potential by a charging member, such as a charging roller, and then an exposure device scans the image carrier with light to form an electrostatic latent image with the charge attenuated. This electrostatic latent image is then developed by a developing device and visualized as a toner image. In such image forming apparatuses, it is necessary to stabilize the surface potential of the image carrier to prevent image defects such as image fogging.

[0003] In addition, in a two-component development device that uses a two-component developer containing a carrier and a toner, the toner in the developer is consumed during the development process, while the carrier remains in the development device. Therefore, the carrier that is stirred together with the toner in the developer container deteriorates as the stirring frequency increases, and as a result, the carrier's ability to impart charge to the toner gradually decreases, causing a decrease in image density and fogging, resulting in a decrease in image quality.

[0004] Patent Document 1 discloses an electrophotographic device that includes a circuit that forms a pulsed electrostatic potential pattern on a photosensitive member and detects the current that flows when this electrostatic potential pattern is developed, a circuit that obtains the surface potential on the photosensitive member by converting the detected current into a surface potential based on a predetermined relational expression or using a pre-created correlation table, and a circuit that feeds back the obtained surface potential to control the charging of the photosensitive member by a charger.

[0005] Patent Document 2 discloses an image forming apparatus that forms multiple patch images on a photosensitive drum, detects the density of the patch images transferred onto an intermediate transfer belt using a density detection sensor, calculates the charge amount distribution and work function distribution of the toner based on the detection results, and determines the deterioration level of the developer using the relationship between the deterioration rank of the developer and the work function distribution that is stored in advance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-295540 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-205712 Summary of the Invention [Problem to be solved by the invention]

[0007] In the method of Patent Document 1, a pulsed electrostatic potential pattern is formed on a photoconductor, a bias is applied to a developing roller, and the surface potential on the photoconductor is obtained by measuring the current flowing from the photoconductor to the developing roller when the electrostatic potential pattern is developed. However, this method has problems in that it is easily affected by the deterioration of the photoconductor and charging member over time, is unstable, and is prone to include errors.

[0008] The method of Patent Document 2 estimates the state of the developer from the toner charge amount. However, although there is a correlation between the toner charge amount and the state of the developer, the extent to which the toner charge amount affects image quality can only be predicted, which is not sufficient for maintaining high image quality.

[0009] SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide an image forming apparatus that can accurately estimate the deterioration state of a two-component developer using a simple method. [Means for solving the problem]

[0010] In order to achieve the above object, a first aspect of the present invention is an image forming apparatus including an image carrier, a charging device, a developing device, a developing voltage power supply, a charging voltage power supply, a current detection unit, and a control unit. The image carrier has a photosensitive layer formed on its surface. The charging device has a charging member that charges the surface of the image carrier to a predetermined surface potential. The developing device has a developer carrier that carries a two-component developer containing a carrier and a toner, and forms a toner image by adhering the toner to an electrostatic latent image formed by exposing the image carrier charged by the charging device. The developing voltage power supply applies a developing voltage including a DC voltage and an AC voltage to the developer carrier. The charging voltage power supply applies a charging voltage to the charging member. The current detection unit detects the developing current flowing between the image carrier and the developer carrier. The control unit controls the developing voltage power supply and the charging voltage power supply. The control unit detects the development current when a development voltage is applied to the developer carrier while changing the DC voltage between two different levels, while the image carrier is charged to a predetermined surface potential by the charging member, using the current detection unit, and executes a developer deterioration estimation process to estimate the degree of deterioration of the two-component developer based on the amount of change in the development current between the two levels. [Effects of the Invention]

[0011] According to the first aspect of the present invention, the deterioration state of the two-component developer can be accurately estimated by utilizing the difference in the amount of change in the development current when a development voltage is applied to a developer carrier while changing the DC voltage between two different levels. Therefore, image defects such as image fogging caused by the deterioration of the two-component developer can be effectively suppressed. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a side cross-sectional view showing the internal configuration of an image forming apparatus 100 according to an embodiment of the present invention. [Figure 2] 1 is a partially enlarged view of the image forming unit Pa and its surroundings, including a control path of the image forming apparatus 100; [Figure 3] A diagram showing the relationship between the DC voltage Vdc of the developing voltage and the developing current [Figure 4]A diagram showing the change in development current when only DC voltage Vdc is applied as the development voltage and when a new developer and a developer that has deteriorated due to durability are used. [Figure 5] FIG. 10 is a diagram showing the change in development current when a DC voltage Vdc and an AC voltage Vac are applied as development voltages and when a new developer and a developer that has deteriorated due to durability are used. [Figure 6] 1 is a flowchart showing an example of developer deterioration prediction and surface potential estimation control in the image forming apparatus 100 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] [1. Configuration of image forming device] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a cross-sectional view showing the internal structure of an image forming apparatus 100 according to one embodiment of the present invention. Within the main body of the image forming apparatus 100 (here, a color printer), four image forming units Pa, Pb, Pc, and Pd are arranged in this order from the upstream side in the transport direction (left side in Fig. 1). These image forming units Pa to Pd are provided corresponding to images of four different colors (yellow, magenta, cyan, and black), and sequentially form images of yellow, cyan, magenta, and black through the processes of charging, exposure, development, and transfer, respectively.

[0014] Each of the image forming stations Pa through Pd is provided with photosensitive drums (image carriers) 1a, 1b, 1c, and 1d, which carry visible images (toner images) of each color. An intermediate transfer belt 8, which rotates counterclockwise in FIG. 1 by a belt drive motor (not shown), is provided adjacent to each of the image forming stations Pa through Pd. The toner images formed on the photosensitive drums 1a through 1d are sequentially transferred (primary transfer) onto the intermediate transfer belt 8, which moves while contacting the photosensitive drums 1a through 1d, and superimposed on each other. The toner images primarily transferred onto the intermediate transfer belt 8 are then secondarily transferred onto a transfer sheet P (an example of a recording medium) by a secondary transfer roller 9. The transfer sheet P, onto which the toner images have been secondarily transferred, is then fixed in a fixing unit 13 and ejected from the image forming apparatus 100. While the photosensitive drums 1a through 1d rotate clockwise in FIG. 1, an image formation process is performed on each of the photosensitive drums 1a through 1d.

[0015] The transfer paper P onto which the toner image is secondarily transferred is stored in a paper cassette 16 located at the bottom of the main body of the image forming apparatus 100, and is transported via a paper feed roller 12a and a pair of registration rollers 12b to the nip between the secondary transfer roller 9 and the drive roller 11 of the intermediate transfer belt 8. A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a seamless belt is usually used. In addition, a belt cleaning device 19 is located downstream of the secondary transfer roller 9 to remove toner and other particles remaining on the surface of the intermediate transfer belt 8.

[0016] Next, the image forming units Pa to Pd will be described. Around and below the rotatably arranged photosensitive drums 1a to 1d, there are provided charging devices 2a, 2b, 2c, and 2d that charge the photosensitive drums 1a to 1d, an exposure device 5 that exposes image information onto each of the photosensitive drums 1a to 1d, developing devices 3a, 3b, 3c, and 3d that form toner images on the photosensitive drums 1a to 1d, and cleaning devices 7a, 7b, 7c, and 7d that remove toner (residual toner) remaining on the photosensitive drums 1a to 1d.

[0017] When image data is input from a host device such as a personal computer, the surfaces of the photosensitive drums 1a-1d are first uniformly charged by the charging devices 2a-2d. Next, the exposure device 5 irradiates light according to the image data, forming electrostatic latent images on the photosensitive drums 1a-1d in accordance with the image data. The developing devices 3a-3d are filled with a predetermined amount of two-component developer containing yellow, magenta, cyan, and black toner, respectively. If the toner content in the two-component developer filled in each developing device 3a-3d falls below a specified value due to the formation of a toner image (described below), toner is replenished from toner containers 4a-4d to each developing device 3a-3d. The toner in the developer is supplied to the photosensitive drums 1a-1d by the developing devices 3a-3d and electrostatically adheres to the photosensitive drums 1a-1d, forming a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 5.

[0018] Then, primary transfer rollers 6a-6d apply an electric field at a predetermined transfer voltage between the primary transfer rollers 6a-6d and the photosensitive drums 1a-1d, and the yellow, magenta, cyan, and black toner images on the photosensitive drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. These four color images are formed in a predetermined positional relationship for forming a predetermined full-color image. After that, toner and other substances remaining on the surfaces of the photosensitive drums 1a-1d after the primary transfer are removed by cleaning devices 7a-7d in preparation for the subsequent formation of a new electrostatic latent image.

[0019] The intermediate transfer belt 8 is stretched over a driven roller 10 on the upstream side and a drive roller 11 on the downstream side. When the intermediate transfer belt 8 starts to rotate counterclockwise as the drive roller 11 is rotated by a belt drive motor (not shown), the transfer paper P is transported from the registration roller pair 12b to a nip portion (secondary transfer nip portion) between the drive roller 11 and the adjacent secondary transfer roller 9 at a predetermined timing, where the toner image on the intermediate transfer belt 8 is secondarily transferred onto the transfer paper P. After the secondary transfer, any toner remaining on the surface of the intermediate transfer belt 8 is removed by a belt cleaner 19. The transfer paper P onto which the toner image has been secondarily transferred is transported to a fixing unit 13.

[0020] The transfer paper P transported to the fixing unit 13 is heated and pressurized by the fixing roller pair 13a, and the toner image is fixed to the surface of the transfer paper P, forming a predetermined full-color image. The transfer paper P on which the full-color image has been formed is then transported in different directions by the branching unit 14, which branches into multiple directions, and is then discharged directly (or after being sent to the double-sided transport path 18 and having images formed on both sides) onto the discharge tray 17 by the discharge roller pair 15.

[0021] [2. Configuration of the image forming unit and its peripherals, including the control path of the image forming device] 2 is a partially enlarged view of the periphery of image forming unit Pa, including the control paths of image forming apparatus 100. In the following explanation, the configuration and control paths of image forming unit Pa will be described, but the configurations and control paths of image forming units Pb to Pd are similar, so their explanation will be omitted.

[0022] The developing device 3a includes a developing roller 30 that carries a two-component developer therein. The developing roller 30 is connected to a developing voltage power supply 43 that generates an oscillating voltage in which a DC voltage Vdc and an AC voltage Vac are superimposed.

[0023] The developing voltage power supply 43 includes an AC constant voltage power supply 43a and a DC constant voltage power supply 43b. During image formation, the developing voltage power supply 43 outputs a developing voltage in which an AC voltage Vac is superimposed on a DC voltage Vdc from the AC constant voltage power supply 43a and the DC constant voltage power supply 43b. The AC constant voltage power supply 43a outputs a sine wave AC voltage Vac generated from a low voltage DC voltage modulated into a pulse shape using a step-up transformer (not shown). The DC constant voltage power supply 43b outputs a DC voltage Vdc obtained by rectifying the sine wave AC voltage Vac generated from a low voltage DC voltage modulated into a pulse shape using a step-up transformer.

[0024] The current detection unit 44 detects the development current flowing between the photosensitive drum 1a and the development roller 30. The development current flowing to the current detection unit 44 may be caused by the movement of toner between the photosensitive drums 1a to 1d and the development roller 30, or may be caused by the movement of carrier.

[0025] The charging voltage power supply 45 applies a charging voltage consisting of a DC voltage to the charging roller 34 of the charging device 2a. The configuration of the charging voltage power supply 45 is similar to that of the DC constant voltage power supply 43b of the developing voltage power supply 43.

[0026] Transfer voltage power supply 47 applies a primary transfer voltage and a secondary transfer voltage of opposite polarity (negative polarity) to the toner to primary transfer rollers 6a to 6d and secondary transfer roller 9 (see FIG. 1), respectively. In this embodiment, constant current control is used to apply to primary transfer rollers 6a to 6d and secondary transfer roller 9 a transfer voltage that causes a constant current (transfer current) of opposite polarity (negative polarity) to the toner to flow through primary transfer rollers 6a to 6d and secondary transfer roller 9.

[0027] The cleaning device 7a includes a collection roller 31 that temporarily holds the residual toner remaining on the surface of the photosensitive drum 1a. The collection roller 31 is connected to a cleaning voltage power supply 48 that generates a DC voltage. The cleaning voltage power supply 48 applies a voltage of the same polarity (positive polarity) as that of the toner and a voltage of the opposite polarity (negative polarity) to the collection roller 31. The configuration of the cleaning voltage power supply 48 is similar to that of the DC constant voltage power supply 43b.

[0028] Next, the control system of image forming apparatus 100 will be described with reference to Fig. 2. Image forming apparatus 100 is provided with a main control unit 80 constituted by a CPU and the like. Main control unit 80 is connected to a storage unit 70 constituted by a ROM, RAM and the like. Main control unit 80 controls each unit of image forming apparatus 100 (charging devices 2a-2d, developing devices 3a-3d, exposure device 5, primary transfer rollers 6a-6d, cleaning devices 7a-7d, secondary transfer roller 9, fixing unit 13, developing voltage power supply 43, charging voltage power supply 45, transfer voltage power supply 47, cleaning voltage power supply 48, voltage control unit 50, etc.) based on the control program and control data stored in storage unit 70.

[0029] Based on control signals sent from the main control unit 80, the voltage control unit 50 controls a development voltage power supply 43 that applies a development voltage to the development roller 30, a charging voltage power supply 45 that applies a charging voltage to the charging roller 34, a transfer voltage power supply 47 that applies a transfer voltage to the primary transfer rollers 6a to 6d and the secondary transfer roller 9, and a cleaning voltage power supply 48 that applies a voltage to the collection roller 31. The voltage control unit 50 may be configured by a control program stored in the storage unit 70.

[0030] The internal temperature and humidity sensor 60 constantly detects the temperature and relative humidity inside the image forming apparatus 100, specifically in the vicinity of the image forming units Pa to Pd. The detected temperature and humidity are sent to the main control unit 80.

[0031] The main control unit 80 is connected to a liquid crystal display unit 90 and a transmission / reception unit 91. The liquid crystal display unit 90 functions as a touch panel for the user to make various settings for the image forming apparatus 100, and also displays the status of the image forming apparatus 100, the image formation status, the number of printed pages, etc. The transmission / reception unit 91 communicates with the outside world using a telephone line or an internet line. The transmission / reception unit 91 functions as an input unit that receives print commands and image data from a higher-level device such as a personal computer.

[0032] [3. Estimation of the degree of developer deterioration and the surface potential of the photosensitive drum] The following describes a method for estimating the degree of deterioration of the developer in the developing devices 3a-3d and the surface potential of the photosensitive drums 1a-1d, which are characteristic features of the present invention. First, the method for estimating the surface potential of the photosensitive drums 1a-1d will be described. Figure 3 is a graph showing the relationship between the DC voltage Vdc of the developing voltage and the developing current.

[0033] As shown in Figure 3, when no DC voltage (0 [V]) is applied to the developing roller 30 and the charging roller 34, a small amount of charged toner moves from the developing roller 30 to the photosensitive drums 1a-1d. This movement is detected as a positive uncharged developing current. This uncharged developing current is defined as the reference current Cs (Vdc = 0 [V], surface potential = 0 [V]).

[0034] Then, with a predetermined charging voltage applied to the charging roller 34 to charge the photosensitive drums 1a to 1d to a predetermined surface potential, the DC voltage Vdc is gradually changed (increased) from an initial value smaller than the surface potential V0, and the progress of the development current detected by the current detection unit 44 is measured.

[0035] When the DC voltage Vdc becomes equal to the surface potential of the photosensitive drums 1a to 1d (ΔV=0 [V]), only the uncharged developing current (reference current Cs) flows between the developing roller 30 and the photosensitive drums 1a to 1d. That is, the DC voltage Vdc1 at the point where the curve L representing the change in the developing current intersects with the reference current Cs becomes equal to the surface potential of the photosensitive drums 1a to 1d, and therefore the surface potential can be estimated to be Vdc1.

[0036] Although the accuracy is slightly lower than the above method, in order to simplify the estimation procedure, the DC voltage Vdc when the development current flowing from the development roller 30 to the photosensitive drums 1a to 1d becomes 0 [V] may be estimated as the surface potential without obtaining the reference current Cs.

[0037] Next, a method for estimating the degree of deterioration of a developer will be described. Fig. 4 is a diagram showing the change in development current when only a DC voltage Vdc is applied as the development voltage and when a new developer (hereinafter referred to as a new developer) and a developer that has deteriorated due to durability (hereinafter referred to as a durable developer) are used. In Fig. 4, the change in development current when a new developer is used is shown by a solid line. The change in development current when a durable developer is used is shown by a dashed line.

[0038] As shown in Figure 4, comparing the change in development current (change after durability) Δc1 for durable developer and the change in development current (initial change) Δc2 for new developer from point a to point b, we can see that Δc1 is larger than Δc2. The reason for this is that as developer deteriorates, the carrier's ability to impart charge to the toner decreases, weakening the force that attracts the toner and carrier. Therefore, when the development voltage is lower than the surface potential, the carrier tends to move to the photosensitive drums 1a-1d, and conversely, when the development voltage is higher than the surface potential, the toner tends to move to the photosensitive drums 1a-1d.

[0039] Therefore, the deterioration state of the developer can be estimated by utilizing the difference in the amount of change in the development current. For example, a threshold value for the amount of change in the development current is set in advance based on experiments or simulations, and it is determined that the developer has deteriorated when the amount of change in Δc1 is equal to or greater than the threshold value, thereby making it possible to estimate the current deterioration state of the developer.

[0040] Furthermore, by acquiring the amount of change Δc2 in the development current with new agents when new developing devices 3a to 3d are first used, the amount of change Δc1 in the development current with durable agents can be compared with Δc2. Specifically, when the difference Δc2 - Δc1 in the amount of change in the development current between new agents and durable agents is equal to or greater than a predetermined threshold, it is determined that the developer has deteriorated. This enables more accurate determination and estimation of the deterioration state of the developer, regardless of variations in the measurement of the development current.

[0041] In the above method, the amount of toner movement (fog toner amount) to the photosensitive drums 1a-1d, a phenomenon that affects image quality, is directly expressed as a numerical value, i.e., the amount of change in the development current. Therefore, it is easier to understand how this affects image quality compared to a method that measures the toner charge amount and indirectly estimates developer deterioration from the measured toner charge amount. Furthermore, as with estimating the surface potential, the state of developer deterioration is determined from the development current, so that not only the state of the developer but also the surface potential can be estimated at the same time. Note that the estimation of the state of developer deterioration and the estimation of the surface potential do not necessarily need to be performed simultaneously; they can also be performed separately.

[0042] Incidentally, the difference in the amount of change in development current between a durable developer and a new developer described above does not occur when a development voltage including an AC voltage Vac is applied. The reason for this is that applying an AC voltage Vac causes the toner to move back and forth between the photosensitive drums 1a-1d and the development roller 30, which has the effect of reducing fogging. Therefore, the deterioration state of the developer can also be estimated in the same way as above, depending on whether the development voltage includes or does not include the AC voltage Vac.

[0043] Fig. 5 shows the change in development current when a DC voltage Vdc and an AC voltage Vac are applied as development voltages and a new agent and a durable agent are used. In Fig. 5, the change in development current when a new agent is used is shown by a solid line. The change in development current when a durable agent is used is shown by a dashed line.

[0044] As shown in Figure 5, when comparing the change in development current Δc1 for durable agent and the change in development current Δc2 for new agent from point a to point b, it can be seen that there is almost no difference between Δc1 and Δc2. In other words, the difference in the change in development current depending on whether the development voltage includes or does not include the AC voltage Vac changes between new agent and durable agent.

[0045] Therefore, the deterioration state of the developer can be estimated by utilizing the difference in the amount of change in the development current depending on whether or not the AC voltage Vac is included. For example, when starting to use new developing devices 3a to 3d, the amount of change in the development current (first initial change) Δc2 when the AC voltage Vac is not included with new developer and the amount of change in the development current (second initial change) Δc2' when the AC voltage Vac is included are measured, and the difference in the amounts of change Δc2 - Δc2' is recorded.

[0046] Next, at a predetermined timing after the start of use of the developing devices 3a-3d, the change in development current (change during first durability test) Δc1 when the AC voltage Vac is not included and the change in development current (change during second durability test) Δc1' when the AC voltage Vac is included are measured, and the difference in the change amounts Δc1-Δc1' is compared with the difference Δc2-Δc2' when the developer is new. If the difference between Δc2-Δc2' and Δc1-Δc1' is equal to or greater than a predetermined threshold, the developer is determined to have deteriorated. This enables a more accurate determination of the deterioration state of the developer, regardless of variations in the development current measurements.

[0047] Furthermore, even without measuring the amounts of change Δc2 and Δc2' in the development current when using new developer, the state of deterioration of the developer may be estimated based only on the difference Δc1-Δc1' between the amounts of change Δc1 and Δc1' in the development current when the development voltage when using durable developer includes and does not include an AC voltage. For example, a threshold value for the difference Δc1-Δc1' in the amounts of change in the development current may be determined in advance, and control may be performed to determine that the developer has deteriorated when Δc1-Δc1' is equal to or greater than the threshold value.

[0048] Furthermore, since the deterioration state of the developer is estimated using the development current in the same way as the estimation of the surface potential, it is possible to estimate not only the deterioration state of the developer but also the surface potential at the same time. Note that the estimation of the deterioration state of the developer and the estimation of the surface potential do not necessarily have to be performed simultaneously, and they may be performed separately.

[0049] 6 is a flowchart showing an example of developer deterioration prediction and surface potential estimation control in image forming apparatus 100 of the present invention. The procedure for estimating the deterioration state and surface potential of developer will be described along the steps of FIG. 6, with reference to FIGS. 1 to 4 as necessary.

[0050] First, the main control unit 80 determines whether it is developer deterioration prediction timing (step S1). If it is not developer deterioration prediction timing (No in step S1), the process ends without predicting developer deterioration.

[0051] When it is the deterioration prediction timing of the developer, such as when the cumulative number of printed sheets since the start of use of the developing devices 3a to 3d or the cumulative number of printed sheets since the previous deterioration prediction reaches a predetermined number of sheets (Yes in step S1), the change amount Δc1 of the developing current when different DC voltages Vdc (points a and b in FIG. 4) are applied to the developing roller 30 is measured (step S2).

[0052] Next, the main control unit 80 compares with the change amount Δc2 of the developing current measured by applying different DC voltages Vdc (points a and b in FIG. 4) to the developing roller 30 with a new developer, which is stored in the storage unit 70 (step S3), and determines whether Δc2 - Δc1 is equal to or greater than a predetermined value A (step S4). When Δc2 - Δc1 ≥ A (Yes in step S4), since it is predicted that the deterioration of the developer has advanced to a level where replacement is necessary, a recovery operation of the developer is executed (step S5).

[0053] Specifically, a forced discharge operation is executed in which the developing voltage is applied to the developing rollers 30 of the developing devices 3a to 3d in a state where the photosensitive drums 1a to 1d are not charged, and the toner carried on the developing rollers 30 is forcibly discharged onto the photosensitive drums 1a to 1d. The forcibly discharged toner is collected by the cleaning devices 7a to 7d. Alternatively, when the developing devices 3a to 3d have a developer discharge portion capable of discharging surplus developer, a new carrier is supplied together with the toner from the toner containers 4a to 4d, and the deteriorated toner and carrier in the developing devices 3a to 3d are forcibly discharged.

[0054] On the other hand, when Δc2 - Δc1 < A in step S4 (No in step S4), since it is predicted that the deterioration of the developer has not advanced to a level where replacement is necessary, the process proceeds to the next step without performing the recovery operation of the developer.

[0055] Next, the main control unit 80 estimates the surface potential of the photosensitive drums 1a to 1d (step S6). Specifically, the DC voltage Vdc1 (see FIG. 3) at the intersection point P of the transition of the developing current measured in step S2 and the reference current Cs measured in advance is estimated as the surface potential.

[0056] Based on the result of the estimation of the surface potential, the main control unit 80 determines whether or not adjustment of the surface potential is necessary (step S7). If it is determined that the estimated surface potential is lower than the target value and that adjustment of the surface potential is necessary (Yes in step S7), the main control unit 80 corrects the charging voltage applied to the charging roller 34 so that the surface potential becomes the target value (step S8). If it is determined that adjustment of the surface potential is not necessary (No in step S7), the process ends without correcting the charging voltage.

[0057] 6, the state of the developer is estimated based on the amount of change in the development current measured at the start of use of the developing devices 3a to 3d and the amount of change in the development current measured at a predetermined timing after the start of use of the developing devices 3a to 3d, thereby making it possible to more accurately estimate the amount of fogging toner generated due to the deterioration state of the developer than in the past.As a result, image defects such as image fogging can be effectively suppressed.

[0058] Furthermore, by detecting the development current flowing between the photosensitive drum 1a and the development roller 30 and estimating the surface potential based on the detected development current, the surface potential can be measured with high accuracy. Furthermore, a dedicated potential measuring device for measuring the surface potential is not required, which contributes to reducing the cost of the image forming apparatus 100. Furthermore, since the degree of deterioration of the developer and the surface potential can be estimated simultaneously, the cost and machine size can be reduced.

[0059] Furthermore, if the degree of developer deterioration has progressed to the point where replacement is necessary, a developer recovery operation is performed, eliminating the risk of continuing printing operations with developer deterioration in an advanced state. Note that, although the state of developer deterioration is determined using the method of Figure 4 in which only DC voltage Vdc is applied as the development voltage in Figure 6, the method of Figure 5 in which the state of developer deterioration is determined based on the difference in the amount of change in development current when the development voltage includes AC voltage Vac and when it does not include AC voltage Vac may also be used.

[0060] In addition, the present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, in the above-described embodiment, the photosensitive drums 1a to 1d are charged to a predetermined surface potential V0, and the DC voltage Vdc is varied from an initial value smaller than the surface potential V0 to measure the change in the development current, and the DC voltage Vdc at the point where the curve L recording the change in the development current intersects with the reference current Cs is estimated as the surface potential of the photosensitive drums 1a to 1d. However, for example, the development current may be measured at only two points, and the DC voltage Vdc at the point where the line passing through the two points intersects with the reference current may be estimated as the surface potential of the photosensitive drums 1a to 1d.

[0061] 1 has been described as an example of image forming apparatus 100, but the present invention is not limited to color printers and may be applied to other image forming apparatuses that charge photosensitive drums 1a-1d using a charging member such as charging roller 34, such as color copiers, digital multifunction peripherals, and facsimiles.The effects of the present invention will be described more specifically below with reference to examples. [Example]

[0062] [Determination of the degree of deterioration of developer using only DC voltage as the development voltage] An evaluation was made by applying only a DC voltage Vdc as a development voltage and determining the degree of deterioration of the developer based on the amount of change in development current.

[0063] The image formation conditions were a printing speed (process speed) of 55 sheets per minute, and the developing roller 30 used a developing sleeve 31 with an outer diameter of 20 mm, the outer surface of which was knurled (80 rows circumferentially) and blasted. The regulating blade 27 used a magnetic blade made of stainless steel (SUS430) with a thickness of 1.5 mm. A developing voltage was applied to the developing roller 30, which was a DC voltage Vdc superimposed with a square-wave AC voltage of 1200 V peak-to-peak (Vpp), 50% duty, and 8 kHz frequency.

[0064] The photosensitive drums 1a to 1d were positively charged organic photosensitive drums (OPC: Organic Photo Conductors) with a 32 μm-thick organic photosensitive layer formed as the photosensitive layer, the peripheral speed ratio of the developing roller 30 to the photosensitive drums 1a to 1d was 1.8 (trail rotation at the opposing position), and the distance between the photosensitive drums 1a to 1d and the developing roller 30 (DS distance) was 0.30 mm. The charging roller 34 was a rubber roller with a conductive rubber layer 3 mm thick and a volume resistivity of 6.0 [LogΩ] laminated on the outer surface of a core metal with an outer diameter of 6 mm, and a charging voltage consisting only of a DC voltage was applied.

[0065] The toner used was a positively charged toner with an average particle diameter of 6.8 μm, and the carrier used was a ferrite-resin coated carrier with an average particle diameter of 38 μm. The initial toner concentration in the developer (weight ratio of toner to carrier) was 6%.

[0066] The test method was as follows: first, as shown in Figure 4, new developer was used and two different levels of DC voltage Vdc (points a and b) were applied, and the change in development current Δc2 was measured. Next, after durability printing, the same two levels of DC voltage Vdc (points a and b) as used when measuring Δc2 were applied, and the change in development current Δc1 was measured. The changes in development current Δc2 and Δc1 are shown in Table 1.

[0067] [Table 1]

[0068] As shown in Table 1, the amount of change in development current Δc1 for durable developer is nearly three times the amount of change in development current Δc2 for new developer. Based on this amount of change in development current, a threshold value for the difference in change Δc2 - Δc1 is determined, and when Δc2 - Δc1 exceeds the threshold value, it can be determined that the developer has deteriorated. [Example]

[0069] [Determination of the Deterioration Level of Developer Using DC and AC Voltages as the Development Voltage] An evaluation was performed to determine the degree of deterioration of the developer based on the amount of change in development current when only DC voltage Vdc was applied as the development voltage and when DC voltage Vdc and AC voltage Vac were applied.

[0070] The test method was as follows: first, a new developer was used as the developer, and the amount of change in development current Δc2 when two different levels of DC voltage Vdc (points a and b) were applied was measured, as in Example 1. After durability printing, the amount of change in development current Δc1 when the same two levels of DC voltage Vdc (points a and b) as those used in measuring Δc1 were applied was measured (Table 1).

[0071] Next, the changes in development current, Δc2' and Δc1', were measured when a DC voltage Vdc and an AC voltage Vac were applied. Table 2 shows the changes in development current, Δc2' and Δc1', when a DC voltage Vdc and an AC voltage Vac were applied. Table 3 also shows the differences in the changes in development current, Δc2-Δc2' and Δc1-Δc1', between when an AC voltage Vac was not included (Table 1) and when an AC voltage Vac was included (Table 2).

[0072] [Table 2]

[0073] [Table 3]

[0074] As shown in Table 2, when the development voltage includes an AC voltage Vac, there is almost no difference between the amounts of change in development current Δc2' and Δc1' between new and durable agents. Therefore, as shown in Table 3, the difference Δc2-Δc2' in the amount of change in development current when an AC voltage Vac is included and not included for new agents is greater than the difference Δc1-Δc1' in the amount of change in development current when an AC voltage Vac is included and not included for durable agents.

[0075] The deterioration state of the developer can be determined based on the difference between Δc2-Δc2' and Δc1-Δc1'. For example, the amounts of change Δc2 and Δc2' in the development current when a new developer is used with and without an AC voltage are measured, and the difference in the amounts of change Δc2-Δc2' is stored in the storage unit 70. Then, the amounts of change Δc1 and Δc1' in the development current when an AC voltage is used and without an AC voltage are measured as needed, and the difference in the amounts of change Δc1-Δc1' is compared with Δc2-Δc2', thereby making it possible to estimate the deterioration state of the developer regardless of variations in the measurement of the development current.

[0076] From the results of Examples 1 and 2, it was confirmed that the degree of deterioration of the developer can be determined with high accuracy based on the amount of change in the development current. [Industrial Applicability]

[0077] The present invention can be applied to an electrophotographic image forming apparatus, and by using the present invention, it is possible to provide an image forming apparatus that can accurately detect the degree of deterioration of a developer by a simple method. [Explanation of symbols]

[0078] 1a to 1d Photosensitive drum (image carrier) 2a~2d Charging device 3a~3d developing device 5 Exposure equipment 6a~6d Primary transfer rollers 8 Intermediate transfer belt 9 Secondary transfer roller 19 Belt cleaner 30 Developing roller (developer carrier) 34 Charging roller (charging member) 43 Development voltage power supply 44 Current detection unit 45 Charge voltage power supply 47 Transfer voltage power supply 48 Cleaning voltage power supply 50 Voltage control section 70 Memory section 80 Main control unit (control unit) 90 Liquid crystal display unit (notification unit) 100 image forming device

Claims

1. an image carrier having a photosensitive layer formed on its surface; a charging device having a charging member for charging the surface of the image bearing member to a predetermined surface potential; a developing device having a developer carrier that carries a two-component developer containing a carrier and a toner, and that forms a toner image by attaching the toner to an electrostatic latent image formed by exposing the image carrier that has been charged by the charging device; a developing voltage power source that applies a developing voltage including a DC voltage and an AC voltage to the developer carrier; a charging voltage power source that applies a charging voltage to the charging member; a current detection unit that detects a development current flowing between the image carrier and the developer carrier; a control unit that controls the developing voltage power supply and the charging voltage power supply; In an image forming apparatus comprising: The control unit an image forming apparatus characterized in that a developer deterioration estimation process is executed, in which the image carrier is charged to a predetermined surface potential by the charging member, the current detection unit detects the development current when the development voltage is applied to the developer carrier while changing the DC voltage between two different levels, and the degree of deterioration of the two-component developer is estimated based on the amount of change in the development current between the two levels.

2. The control unit, in the developer deterioration estimation process, At a predetermined timing after the start of use of the developing device, a durability change amount Δc1 is measured, which is a change amount of the developing current when the developing voltage not including the AC voltage is applied to the developer carrier while the DC voltage is changed between two different levels; 2. The image forming apparatus according to claim 1, wherein the two-component developer is determined to be in a deteriorated state when the amount of change Δc1 during durability becomes equal to or greater than a predetermined value.

3. At the start of use of the developing device, an initial change amount Δc2 is measured, which is the amount of change in the developing current when the developing voltage not including the AC voltage is applied to the developer carrier while the DC voltage is changed between two different levels in a state in which the image carrier is charged to the predetermined surface potential, The control unit, in the developer deterioration estimation process, At a predetermined timing after starting use of the developing device, in a state in which the image carrier is charged to the predetermined surface potential, a durability change amount Δc1 is measured, which is the change amount of the development current when the development voltage not including the AC voltage is applied to the developer carrier while changing the DC voltage between the same two levels as when the initial change amount Δc2 was detected; 2. The image forming apparatus according to claim 1, wherein when a difference Δc2-Δc1 between the initial change amount Δc2 and the durability change amount Δc1 becomes equal to or greater than a predetermined value, it is determined that the two-component developer is in a deteriorated state.

4. The control unit, in the developer deterioration estimation process, At a predetermined timing after the start of use of the developing device, while changing the DC voltage between two different levels, a first durability change amount Δc1 which is the change amount of the developing current when the developing voltage not including the AC voltage is applied to the developer carrier, and a second durability change amount Δc1′ which is the change amount of the developing current when the developing voltage including the AC voltage is applied to the developer carrier are measured; 2. The image forming apparatus according to claim 1, wherein when a difference Δc1-Δc1' between the first durability change amount Δc1 and the second durability change amount Δc1' becomes equal to or greater than a predetermined value, it is determined that the two-component developer is in a deteriorated state.

5. At the start of use of the developing device, while the image carrier is charged to the predetermined surface potential, the DC voltage is changed between two different levels, and a first initial change amount Δc2, which is the change amount of the developing current when the developing voltage not including the AC voltage is applied to the developer carrier, and a second initial change amount Δc2′, which is the change amount of the developing current when the developing voltage including the AC voltage is applied to the developer carrier, are measured; The control unit, in the developer deterioration estimation process, At a predetermined timing after starting use of the developing device, in a state in which the image carrier is charged to the predetermined surface potential, while the DC voltage is changed between the same two levels as when the first initial change amount Δc2 and the second initial change amount Δc2′ are detected, a first endurance change amount Δc1 which is the change amount of the development current when the development voltage not including the AC voltage is applied to the developer carrier, and a second endurance change amount Δc1′ which is the change amount of the development current when the development voltage including the AC voltage is applied to the developer carrier are measured; 2. The image forming apparatus according to claim 1, wherein the two-component developer is determined to be in a deteriorated state when a difference Δc2-Δc2' between the first initial change amount Δc2 and the second initial change amount Δc2' and a difference Δc1-Δc1' between the first endurance change amount Δc1 and the second endurance change amount Δc1' become equal to or greater than a predetermined value.

6. The image forming apparatus according to claim 1, characterized in that, when it is determined that the two-component developer is in a deteriorated state, the control unit executes a developer recovery process to recover the deteriorated state of the two-component developer by replacing at least one of the carrier and the toner in the developing device.

7. The control unit the developing current when the developing voltage and the charging voltage are not applied to the developer carrier and the charging member, respectively, is measured and used as a reference current; An image forming apparatus according to any one of claims 1 to 6, characterized in that a surface potential estimation process is performed simultaneously with the developer deterioration estimation process, in which a predetermined charging voltage is applied to the charging member to charge the image carrier to the predetermined surface potential, and the DC voltage when the development current detected by the current detection unit becomes equal to the reference current is estimated to be the surface potential.

Images